1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly to a document feeding type image reading apparatus which is suited to be used as an image input device for a copying machine and a scanner.
2. Description of Related Art
In the art of image reading apparatuses for reading images optically, a platen-set method wherein an image of a document set on a platen glass is read and a sheet-through method wherein an image of a document is read while the document is being fed are adopted separately or at the same time. The sheet-through method is advantageous in its small size, low cost, low noise, high-speed image reading and high printing efficiency. Therefore, the sheet-through method is generally adopted for monochromatic copying machines, and it is demanded that the sheet-through method be adopted for color copying machines also.
In the sheet-through method, an image reading position is fixed on a transparent member (a strip of reading glass), and a reading optical system is designed to have a focus on an imaged-surface of a document fed over the reading glass. Therefore, image reading is apt to be influenced by foreign matters such as dust adhering to the reading glass, and portions shaded by the foreign matters are read as image noise, that is, stripes.
The foreign matters stuck on the reading glass are generally sticky matters, such as gluey cracks of an adhesive tape or glue, correction liquid, inkblots and eraser rubbish which peeled off from documents, and these foreign matters are apt to stick to the portion of the reading glass where documents are pressed against. These sticky foreign matters do not come off the reading glass easily and cause image noise. Also, floating matters, such as paper rubbish dispersed from documents and dust in the air, are deposited on the reading glass, which causes image noise. In the sheet-through method, in feeding a document, the document is nipped between rollers and is pressed by guide plates. Therefore, the document, which is generally made of paper, has a stress, and necessarily, paper dust falls down from the document.
In order to avoid this trouble, Japanese Patent Laid-Open Publications No. 9-307695 (Reference 1) and No. 11-146140 (Reference 2) suggest that a step be made in a position upstream of the reading glass in a document feeding direction so that a document can be fed without being in contact with the reading glass. In this non-contact method, adherence of sticky foreign matters to the reading glass can be prevented. On the other hand, floating foreign matters are dragged into a traveling document when the document comes into contact with the reading glass and are ejected naturally. In the non-contact method, the trailing portion of a traveling document comes into contact with the reading glass because the trailing portion is no longer supported after having passed the step, and floating foreign matters may be dragged into the trailing portion of the document and may be ejected.
In the non-contact method, however, the trailing portion of the document which comes into contact with the reading glass is a small area, and it is difficult to certainly remove floating foreign matters from the periphery of the reading glass. A document is apt to have a curl and/or wrinkles in its trailing portion, and the trailing portion may not be flat. Therefore, the trailing portion of a document does not necessarily come into contact with the reading glass, and floating foreign matters which cannot be removed are deposited on the reading glass.
Japanese Patent Laid-Open Publication No. 2001-223832 (Reference 3) suggests an improvement on the apparatus disclosed by Reference 1. Reference 3 suggests that a catcher member for catching foreign matters be provided between the step and the reading glass. In this structure, troublesome maintenance (changes and cleaning) of the catcher member is necessary. Also, the catcher member cannot be located at the reading position, and it is impossible to catch dust which has fallen onto the reading position.
Regarding a monochromatic reading apparatus, the reproducibility of letters is regarded to be important, and image processing for clear reproduction of letters is carried out by heightening the reproducibility of high-density portions, such as letters, and by lowering the reproducibility of low-density portions. Therefore, in images formed by the monochromatic reading apparatus, image noise due to very small floating foreign matters such as paper dust is not very apparent. Regarding a color image reading apparatus, on the other hand, reading of gradation of a document is regarded to be important because it is important to reproduce the gradation of a photo or the like. Therefore, very small floating foreign matters, which do not become apparent in the monochromatic reading apparatus, are also read, and consequently, noise appears on a reproduced image as narrow stripes.